The present invention relates to the protection of active sites on a semiconductive wafer which is sawed into individual active-site-bearing chips, and more particularly to a method of protecting, during and after sawing, active sites such as micromechanical devices from debris and deleterious substances which are produced by, or are used in, such sawing.
Numerous processes are known for producing plural arrays of active sites in and on a first surface of a semiconductor wafer. Each active site may comprise a micromechanical device having deflectable elements such as a digital micromirror device (DMD) such as that manufactured by Texas Instruments of Dallas Tex., but could also include accelerometers, sensors etc. The wafer is ultimately separated into a plurality of individual chips, also known as dies or bars, each of which includes one of the active site arrays, the array having a xe2x80x9ctopxe2x80x9d surface comprising a portion of what was formerly the wafer""s first surface. Each active site array has associated therewith one or more bond pads on its top surface. The bond pads are rendered selectively electrically continuous with the active sites, typically by depositing or otherwise forming them on top of, and in electrical contact with, conductors formed on the wafer. Some of the same steps used to produce the active sites may also produce the conductors, which are themselves electrically continuous with the active sites.
The separation of the wafer into individual chips is effected by an operation which may be referred to as xe2x80x9ccomplete sawingxe2x80x9d, or xe2x80x9cpartial sawingxe2x80x9d followed by a xe2x80x9cbreakingxe2x80x9d process. Sawing separates the wafer along lines or paths commonly referred to as xe2x80x9cstreetsxe2x80x9d extending between locations whereat adjacent active site arrays reside or will ultimately reside.
Sawing, which typically involves mechanical abrasion and erosion of the wafer, may be achieved by a number of techniques, including those which utilize rotating saw blades and vibrating tips. Accordingly, the act of sawing the wafer itself produces substantial debris which includes small pieces of the wafer including oxide particles from a CMOS layer, and possibly small pieces of the saw blade or vibrating tip. Sawing is also typically accompanied by cooling/lubricating fluids and other substances which prevent the saw blade or tip from damaging the wafer and which prolong the life of the saw blade or tip.
The debris resulting from and the substances used in sawing can degrade the performance of or render inoperative the active sites, particularly moving parts of a micromechanical device. If the active sites include a spatial light modulator (xe2x80x9cSLMxe2x80x9d), such as that known as a deflectable mirror device or a digital micromirror device (collectively xe2x80x9cDMDxe2x80x9d), each active site may be even more sensitive to the effects or presence of the debris and fluids resulting from and used in sawing.
A DMD is a multilayered micromechanical structure formed on a wafer, which includes a light-reflective beam or similar mechanical member. An example of a DMD is disclosed in commonly assigned U.S. Pat. No. 5,061,049 entitled xe2x80x9cSpatial Light Modulator and Methodxe2x80x9d, the teachings incorporated herein by reference. An area or linear array of beams are associated with an active site and are so mounted to, or hinged from, the structure formed on the wafer as to be deflectable or movable between a normal position and other positions. Deflection of the beam may be achieved by electrostatically attracting the beam toward (or to) an underlying adjacent electrode which is at a different electrical potential from that of the beam. Deflection of the beam stores energy in its mount or hinge, which stored energy tends to return the beam to its normal position absent the electrical potential. Movement of the beam, which may be binary or analog, is controlled by the circuit components of the active site associated with the beam and functioning as an addressing circuit. Deflection of the beam is facilitated by an undercut well which underlies the beam. The well is formed by appropriate etching of one of the spacer layers of material deposited on the wafer, typically comprising photoresist.
In use, an array or matrix of DMD""s is arranged to receive light from a source. The received light which is incident on the reflective beams is selectively reflected or not reflected onto a viewing surface, such as a screen, depending on the position of the beams. Such reflected light is directed by each beam onto the viewing surface in only one selected position, which may be the normal position or one of the other deflected positions. In all other positions of each beam other than the selected position, the incident, reflected light is directed in such a way that it does not fall on the viewing surface, such as to a light absorber. Appropriate energization of the circuit components of the addressing circuit associated with each beam of each active site in the array or matrix permits the beam-reflected light on the viewing surface to be presented as a rasterized array of pixels (as in a typical television) or as a scanning line of pixels (as in a line printer). Thus, the beam of each active site is or acts as a pixel.
Because a DMD includes circuit components as well as a microminiature deflectable beam, it is especially sensitive to debris resulting from sawing the wafer and to the fluids and other substances used to facilitate sawing. Such debris can enter the undercut well and prevent deflection of the beam. In one technique, formation of the circuit components of the active sites and etching or other steps which define the beams are followed by the deposit of a protective layer thereon. Sawing of the wafer to separate the arrays then proceeds, the protective layer preventing the sawing operation from damaging the circuit components and the etch-defined beams. After sawing is completed, but while still in wafer form, the protective layer is removed and the undercut wells are then formed under each beam by plasma etching the spacer layer. Formation of the wells at this time reduces the sawing-related debris and substances from entering the wells. However, automatic pick-and-place equipment and other automatic or human handling can still generate damaging particles until the DMD device is secured to and finally hermetically sealed in the package.
One object of the present invention is the provision of a method of protecting each active site in plural active site arrays on a fully processed semiconductor wafer, particularly active sites which include a DMD SLM or other micromachine, so that beam-defining etching and well formation may all be carried out with reduced particles inhibiting operation of the device before it is hermetically sealed in a package.
In commonly assigned U.S. Pat. No. 5,389,182 to Mignardi entitled xe2x80x9cUse of a Saw Frame with Tape as a Substrate Carrier for Wafer Level Backend Processingxe2x80x9d, there is disclosed a method for processing a wafer containing microelectronic mechanical devices. The method allows all fabrication and test steps to be performed in wafer form upon a dicing tape, whereby the wafer is completely sawn to separate the devices from one another, but wherein the devices are left on the dicing tape during the remaining fabrication steps including device testing and undercutting of the spacer layer beneath the mirrors.
In commonly assigned U.S. Pat. No. 5,393,706 to Mignardi, et al entitled xe2x80x9cIntegrated Partial Sawing Processxe2x80x9d, there is disclosed a process for partially sawing streets on a semiconductor wafer, and then covering the streets with a protective material. After the wafer is broken to separate the die from one another, the protective material may or may not be removed. The protective material over the streets traps any debris from the break and prevents it from contaminating the active parts of the micromechanical device. The tape is then peeled away, and the chips are removed by standard semiconductor machines which handle picking and placing of individual chips. The sacrificial layer of photoresist is removed before wafer break, thereby allowing the active elements or micromirrors to move freely.
In commonly assigned U.S. Pat. No. 5,435,876 to Alfaro, et al entitled xe2x80x9cGrid Array Masking Tape Processxe2x80x9d, there is disclosed a method of adhering a tape with an adhesive grid to the partially fabricated wafer. This tape encapsulates each active site beneath a non-adherent protective envelope. The adhesive portions of the tape are registered with the saw streets of the wafer.
In commonly assigned U.S. Pat. No. 5,445,559 to Gale, et al entitled xe2x80x9cWafer-like Processing after Sawing DMD""sxe2x80x9d, there is disclosed a processing fixture and method of fabricating micromechanical devices whereby the wafer is attached to a vacuum fixture after partially sawing the wafer to create saw kerfs. The backside of the wafer is ground down to the saw kerfs to separate the devices. Each device is held on the fixture by a vacuum in the head space above the device. In an alternative embodiment, the devices are separated by sawing completely through the wafer while in the fixture.
In commonly assigned U.S. Pat. No. 5,597,767 to Mignardi, et al entitled xe2x80x9cSeparation of Wafer Into Die with Wafer-Level Processingxe2x80x9d, there is disclosed a two-step die separation process that permits intervening wafer-level processing. Separation lines are inscribed on the top surface of the substantially fabricated wafer. During the inscription step, a protective coating on the wafer protects the top surface of the wafer. The protective layer is then removed after inscription, and the wafer is processed with at least one wafer-level process. This invention allows backend processing and testing to be performed at the wafer level rather than die-by-die.
In commonly assigned U.S. Patent Application Serial No. 60/015,107 entitled xe2x80x9cMethod of Cleaning Wafer After Partial Sawxe2x80x9d, there is disclosed a method of performing a partial-saw to the wafer to form kerfs between the partially fabricated micromechanical devices, whereby a solution of an alkyl glycol and hydrofluoric acid is utilized to clean up debris generated by the partial saw process.
In commonly assigned U.S. Patent Application Serial No. 60/016,732 entitled xe2x80x9cMethod of Reducing Wafer Particles After Partial Sawxe2x80x9d, there is disclosed a method of utilizing compatible layers of photoresist to avoid the generation of microscopic cracks within the layers of photoresist. A very strong hydrofluoric acid can be utilized during the wet clean-up process to remove debris generated during the partial-saw process.
There is desired a method of fabricating and packaging a micromechanical device with reduced particles, while still permitting wafer-level processing to be performed.
The method of the present invention achieves technical advantages by mounting partially fabricated micromechanical devices on a package, and performing the spacer layer undercutting at package level, just prior to attaching a lid to the package. The process of the present invention utilizes wafer-level processing and reduces the chance of particles damaging the device by waiting to undercut the spacer layer until just before the lid is attached to the package. Conventional wafer-level processing is performed up to the point that the partially fabricated devices are separated from one another at the wafer level and then picked and placed on the packages. The present invention avoids any function-inhibiting particles being generated, such as when the devices are undercut after a complete break while still remaining on a wafer tape, but before they are picked and placed on the package.
The present invention comprises a method of fabricating a micromechanical device including the steps of processing a wafer to form a plurality of partially fabricated devices. The devices have a micromechanical structure defined upon a first layer. Thereafter, the wafer is subdivided to separate the partially fabricated devices. Next, these partially fabricated devices are mounted on a package with the first layer still in place. Next, the mounted partially fabricated devices are undercut to remove the first layer from the devices to free the micromechanical structure for movement. Finally, a lid is attached to the package.
Preferably, the undercutting process is performed by a direct isotropic plasma etch process, such as using NF3. This first layer preferably comprises a photoresist material, preferably being UV-cured. The present invention preferably further comprises the step of performing a wire bond between the package and the mounded partially fabricated device before the undercut process. In addition, the device may be passivated after the undercut process before the lid is attached to inhibit sticking between engaging elements. In the preferred embodiment of the present invention, the micromechanical device comprises a beam and a hinge, whereby the hinge supports the beam above a well after the undercutting process. The micromechanical device may be a DMD, although the present invention is also suitable for other micromechanical devices including accelerometers, sensors, and so forth.